Field of the Invention
The present invention relates to a wire electrical discharge machine having a function to correct a machining path in a corner portion.
Description of the Related Art
In wire electrical discharge machining, it is known that a wire electrode deflects due to a discharge repulsion force produced between wire electrode and workpiece or due to turbulence of a machining fluid. When machining is performed along a straight line, the wire electrode deflects in a direction opposite a machining/traveling direction, but this does not adversely affect machining shape.
As shown in FIG. 1, in machining a corner portion of a workpiece 2, if there is no deflection of a wire electrode 3, a machined groove is formed by machining along a machining path 4 in a direction corresponding to a machining direction 6. Actually, however, as shown in FIG. 2, due to a deflection amount 9 of the wire electrode (deflection amount of the wire electrode rearward in the machining/traveling direction), a discrepancy occurs in a corner portion between the machining path 4 and an actual wire trajectory (wire electrode trajectory 12 in the corner portion), resulting in a chipped convex portion 11 or unremoved concave portion 10 on the workpiece 2. In this way, there is a problem in that the corner portion is affected greatly by the deflection amount 9 of the wire electrode, greatly reducing shape accuracy in the concave portion, resulting in a so-called “corner droop,” and making it impossible to obtain a desired shape.
Various measures have been devised to solve such a problem, and the measures are roughly classified into the follows two major types.
(1) A method which reduces deflection of the wire electrode by reducing machining speed and the amount of machining fluid in a corner portion or extending a discharge quiescent time (so-called “machining fluid control” or “energy control”)
(2) A method which corrects a machining path by taking the deflection of the wire electrode into consideration
Of the two measures, the method (2) which corrects a machining path has the advantage of being able to reduce machining time, and some concrete methods have been proposed so far.
Japanese Patent Application Laid-Open No. 61-219529 discloses a controller, comprising: a controller adapted to control an amount of travel of a wire electrode relative to work; a storage adapted to store a deflection amount of the wire electrode on a machined surface of the work; a calculator adapted to successively determine a machining direction of the wire electrode through calculations; and a driver adapted to drive the wire electrode using an amount of correction equal to the deflection amount of the wire electrode.
Japanese Patent Application Laid-Open No. 7-24645 discloses a wire electrical discharge machining apparatus comprising: a controller adapted to control an amount of relative travel of a wire electrode; a corner detector adapted to detect a corner portion in a machining path; and a machining path corrector adapted to sequentially correct a tangential movement over a predetermined distance in the detected corner portion, a movement along the corner portion over a predetermined distance, and an asymptotic return movement.
Japanese Patent Application Laid-Open No. 11-207527 discloses a wire electrical discharge machining apparatus wherein: a first machining path is extended in a corner portion along a tangent in a machining/traveling direction; second and third corrected paths are set at angles larger than a machined corner; and the machining path is corrected so as to return to the original machining path along a fourth path.
However, the technique described in the prior art document described above is proposed as a method for correcting typical convex corners of simple shapes, and when the correction method is applies to a concave corner as shown in FIG. 3, there is a problem in that the wire electrode will bite into a machined surface which is to become a product, producing flaws in the concave corner portion and rendering the machined product into a defective item. Also, even when the correction method is applied to a convex corner, if the convex corner has a shape accompanied by an arc block such as shown in FIG. 11, there is a problem in that machined grooves will intersect each other, producing a portion called a core, as shown in FIG. 14, which can cause machining defects.
Description of corrections to concave corners are provided only by techniques such as disclosed in Japanese Patent Application Laid-Open No. 7-285029. The patent document discloses a method for machining a sharp edge: comprising saving a deflection amount of a wire electrode during wire-cut electrical discharge machining under each machining condition; and correcting a machining path based on the saved deflection amounts so as to make the wire electrode escape by an extra amount in a traveling direction during punching and so as to make a cut during die machining.
The technique disclosed in Japanese Patent Application Laid-Open No. 7-285029 described above proposes a method for correcting a machining path in a concave corner portion by taking die machining as an example, and the method involves making a path correction so as to move the wire electrode in a direction of a line which bisects an angle between a first block (block to be machined first) and a second block (block to be machined second) which form a corner, but the direction of the line does not coincide with such a direction as to cancel off the deflection amount of the wire electrode. Therefore, the larger the angle between the first block and second block, the larger a discrepancy between a moving direction in which the wire electrode is moved to correct the machining path and an instructed direction of the second block. Thus, such a machining path correction method cannot be said to be an effective correction method for concave corners.
Also, when a machining path correction such as shown in FIG. 5 is made (the wire electrode is moved from straight line A-B to straight line B-C to straight line C-B and to straight line B-D) using the technique of Japanese Patent Application Laid-Open No. 7-285029 described above, there is a possibility of damaging an electro-discharge-machined product.
Specifically, as shown in FIG. 2, the deflection of the wire electrode 3 occurs in a direction opposite the machining direction 6 of the wire electrode, causing an actual position 7 of the wire electrode 3 to deviate from a instructed position 8 of the wire electrode 3. When the machining direction 6 of the wire electrode 3 changes, the wire electrode 3 actually advances by taking a shortcut through a corner portion (see the wire electrode trajectory 12 in a corner portion). That is, it is not that the wire electrode 3 deflected rearward in the wire electrode traveling direction follows the wire electrode trajectory 12 with a delay. Thus, when the machining path in FIG. 5 is corrected from point B to point C, the shortcut will create a nick on the product. Also, since the direction of machining path correction coincides with the direction of the line which bisects the angle between the first block and second block which form a corner, there is a problem in that the larger the angle between the first block and second block, the larger the impact of the shortcut, and consequently the deeper the nick.
In this way, Japanese Patent Application Laid-Open No. 7-285029 adopts a method which makes corrections based on the deflection amounts of the wire electrode, but only considers square die machining (in which the angle between the first block and second block is a right angle) and does not propose an appropriate machining method for concave corner portions with an arbitrary angle.